Counterion specificity of surfactants based on dicarboxylic amino acids

The behavior in solution of a series of amino acid-based surfactants having two carboxyl groups separated by a spacer of one, two, or three carbon atoms has been investigated. All three surfactants precipitated on addition of acid, but the aspartate surfactant (with a two-carbon spacer) was considerably more resistant to precipitation than the aminomalonate surfactant (one-carbon spacer) and the glutamate surfactant (three-carbon spacer). The interactions with the monovalent counterions lithium, sodium, and potassium were investigated by conductivity. It was found that lithium ions bound the strongest and potassium ions the weakest to the surfactant micelles. These results were interpreted using the hard and soft acid-base theory. Comparing the three surfactants with respect to binding of one specific counterion, sodium, showed that the aminomalonate surfactant, which has the shortest spacer, bound sodium ions the strongest and the glutamate surfactant, which has the longest spacer, had the lowest affinity for the counterion. Also that could be explained by the hard and soft acid-base concept. The glutamate surfactant was found to be considerably more resistant to calcium ions than the two other surfactants. This was attributed to this surfactant forming an intermolecular complex with the calcium ion at the air-water interface while the aminomalonate and the aspartate surfactants, with shorter distance between the carboxylate groups could form six- and seven-membered intramolecular calcium complexes.